The present invention relates to an integrated light emitting/receiving amplifier element, which is a principal constituent element for optical communication, optical exchange and optical data processing systems.
One of the basic circuits used for optical communication, optical exchange and optical data processing is a light amplifier circuit. Basic study is under way on this kind of circuit, which is expected to permit simpler structure than other conventional electronic circuits. Light amplifier circuits can be classified into combinations of a light receiving element and a semiconductor laser and others whose principal constituent element is a semiconductor laser. A light amplifier circuit of the first category, which is relatively easier to package and assemble and in itself less dependent on temperature can be even more simply structured by integrating the elements.
An example of the integration of a light receiving element and a semiconductor laser is proposed by H. Beneking et al. in "GaAs-GaAlAs Phototransistor/Laser Light Amplifier" published in ELECTRONICS LETTERS, Vol. 16, No. 15, 1980, pp. 602-603. This report proposes, as a method to integrate the two kinds of element, to grow a semiconductor laser on one end face and a phototransistor on the other of a semiconductor substrate. An optical signal incident upon a light receiving face formed on the emitter side of the phototransistor is converted to a photo-current which is amplified in the phototransistor region and is supplied to the semiconductor laser through the common semiconductor substrate. Since about 2,000 negative electrons (or positive holes) are generated per photon in a usual phototransistor, even very weak optical signals can be regenerated and amplified. However, the structure wherein the semiconductor substrate is sandwiched between the semiconductor laser and the phototransistor requires the division of crystalline growth into at least two phases. Therefore, in this fabricating process, grinding of the wafer into the desired wafer thickness is carried out before the second phase of crystalline growth, and the light receiving face, electrodes and so forth are formed after the crystalline growth. As a consequence, there arises the problem of a thinned wafer thickness and fabricating-difficulty. Accordingly there has been a call for a light amplifier element whose structure permits ready crystalline growth and integration by a usual process.